Career
In 1624, he published, at Grenoble, a series of essays entitled Exercitationes paradoxicae adversus Aristoteleos, in which he declared that, although obliged by the nature of his post to teach the philosophy of Aristotle, he had always taken care to point out the weaknesses of Aristotle's system. Instead of continuing his polemic against the Schoolmen--a sort of skirmishing likely to make enemies for him in the Church--he turned to Epicurus and began, in 1626, to plan a work on his life, morals, and teaching. He was convinced that Epicurus' teaching could be made to conform with the current theology, concerning which he avoided committing himself. In 1632 he published observations on the transit of Venus and in 1641 demonstrated experimentally the truth of Galileo's theory of falling bodies. But his main contribution to science was as a philosopher rather than as an experimentalist.
In 1642 Gassendi replied to Descartes' Meditations with a series of objections which constitute a plea for humility in philosophical speculation. In 1645 he was appointed professor of mathematics (astronomy) at the Royal College in Paris. His inaugural lecture (Institutio astronomica; 1647), while not openly declaring for Galileo, shows sympathy for him. In 1647 appeared his De vita et moribus Epicuri, which marks the date from which public opinion changed concerning Epicurus as a moralist. In 1648, suffering from tuberculosis, Gassendi left for Provence. In 1649 he published his Animadversiones in decimum librum Diogenis Laertii (on Epicurus), together with a short summary of Epicurus' philosophy (Syntagma philosophiae Epicuri).
A complete edition of his works appeared in 1658. The first two volumes contain his Syntagma Philosophicum, which proceeds from logic to physics (general science, cosmology, physiology, psychology) and then to ethics. The logic is a method of enquiry, mainly inductive, but it attempts a balance between experience and reason. The physics describes a world composed of atoms and void, created by God, whose existence is shown by the harmony and order of the world. God is the first cause; all the second causes are motion, and motion is a property of the atoms. The most mobile of the atoms compose the souls of animals capable of sensation and "sensitive reason." The human soul is immaterial and accedes to abstract ideas, but only from sense data. Human beings seek for happiness; it may be attained through virtue and the love of God's perfections.
Gassendi respected orthodox teaching, but the essentials of his picture of the world were to become those of eighteenth-century deism. He was the main rival of Descartes as an exponent of the "new philosophy." He influenced Robert Boyle and John Locke and was greatly esteemed by Sir Isaac Newton.